1. Field of the Invention
The present invention relates to serial-connection and parallel-communication fast interface for PLC host and expansion device, especially relates to serial-connection and parallel-communication fast interface for PLC host and expansion device, wherein the expansion device can be optionally and flexibly added with unlimited order and the accessing time can be reduced.
2. Description of Prior Art
The programmable logic-controller (PLC) is generally connected to external equipments or devices through the external I/O terminals thereof and controls the external equipments or devices by editing a program thereof. However, the prior art PLC has external I/O terminals with predetermined pin counts and expansion device is need to expand analog or digital pin counts when the prior art PLC is to be connected to more external equipments or devices.
The prior art PLC is generally connected to expansion device through serial-connection-serial-communication or parallel-connection-parallel-communication. For PLC in serial-connection-serial-communication with the expansion device, the PLC is serially connected to a plurality of expansion devices. When the PLC sends a data-accessing signal for a designated expansion device, each of the expansion devices will receive and process the data accessing signal to examine whether it is the designated expansion device. However, the communication time for the PLC to access the expansion device will be excessively long.
FIGS. 1 and 2 show the schematic diagram for the prior art PLC in parallel-connection-parallel-communication with expansion device. The PLC is connected through a fixed base 1 and a plurality of connectors 11 with decoding function. The connectors 11 are connected to the bus 15 of the fixed base 1 and comprise address lines, data lines and control lines, whereby the PLC host 12 is connected to a plurality of expansion devices 13 and 14 in parallel fashion. The connector 11 provides decoding function to reduce the communication time for the PLC to access the expansion device. However, the connector 11 has decoding function with fixed order and it places limitation on arrangement order of the expansion devices 13 and 14. The order of the expansion devices 13 and 14 cannot be changed. The fixed base 1 and a plurality of connectors 11 are needed even when the PLC host 12 is connected to one or just a few of expansion devices 13.